This invention relates to a method and apparatus for two-sided coating of elongated strip articles, such as metal sheet or strip. More particularly, the invention relates to such coating achieved by the utilization of direct coating apparatus having floating extrusion heads facing opposite sides of the sheet or strip articles to be coated.
Direct coating of strip articles with layers of coating materials utilizing two-sided direct coaters is known, for example, from U.S. Pat. No. 5,807,434 to Robert A. Innes (hereinafter xe2x80x9cthe Innes Patentxe2x80x9d), issued Sep. 15, 1998, and assigned to Alcan International Limited. This patent is concerned with two-sided sheet article coating utilizing the concept of xe2x80x9cfloatingxe2x80x9d coating heads directly opposing each other on opposite sides of the strip article to be coated. Each coating head has an elongated slot extending across the width of the strip article generally at right angles to the direction of advancement of the strip article through the coating apparatus. The slot allows solidifiable liquid coating material to be delivered into the gap formed between each coating head and the adjacent surface of the strip article to be coated. On the downstream side of the slot of each coating head (i.e. downstream relative to the direction of strip advancement), an extended, generally flat, sloping surface (referred to as a xe2x80x9clandxe2x80x9d) is provided. This land slopes with a predetermined angle inwardly towards the surface to be coated in the direction of advancement of the strip article. The gap into which the coating material is delivered consequently narrows in the direction of strip advancement, and this causes the coating material to be compressed in the gap and to exert an outward force on the land as the material is squeezed to the desired coating film thickness. At least one of the coating heads is movable generally at right angles to the strip article and is urged by some form of pushing arrangement (e.g. hydraulic or pneumatic cylinders, springs, etc.) towards the strip. The outward force generated on the land by the coating material balances the inward force provided by the pushing arrangement pressing the coating head towards the strip article, causing the coating head to float on the newly forming film of coating material without actually touching the strip article itself. This floating effect allows a constant thickness of coating material to be applied to the strip surfaces regardless of the condition of those surfaces, since the floating coating heads follow any contours or irregularities of the strip thickness as the strip article is advanced through the coater apparatus.
Direct coaters of this kind can be used for applying various kinds of solvent-borne coatings (e.g. paints, lacquers, enamels, etc.) or molten solid coatings (e.g. molten polymers, etc.). The thickness of the coating can be controlled by varying certain coating parameters, such as the land (extended surface) width and angle, the effective viscosity of the coating medium, the speed of the strip article through the coating zone, the applied pressure, and the like. The effect of varying these parameters may be quite complex. If the conditions employed for each coating head are the same, the coating thicknesses will be the same on each side of the strip article. However, if different coating thicknesses are required on opposite sides of the strip article, or if the same coating thickness is required on each side despite the use of coating materials having different properties, special steps are required.
This can be important because it is often desirable in commerce to provide different coating materials or coating thicknesses on opposite sides of a strip article. For example, aluminum or other metal sheet material intended for beverage can bodies or can ends typically requires coatings of two microns or less on the side intended for the outside of the can, but (for some beverages, e.g. those containing acids or salts) requires coatings of seven microns or more on the side intended for the inside of the can. The properties of the coating materials on the two opposite sides may also have to be different. For example, inner surfaces generally need to be compatible with food or beverages, while outer surfaces may require durable protection to resist abrasions during product handling.
As disclosed in the Innes patent, the ratio of coating film thickness on the two sides of the strip article can be controlled in two ways, i.e. by providing coating heads with different land angles and/or lengths (in the direction of the strip article advance) on each side of the strip, or by employing different coating formulations, which exhibit different viscosities under the high shear coating conditions, for the two coating heads. Variations in coating speeds cannot be used to produce differences of the thickness ratio (since, in two-sided coaters, both sides of the strip are coated simultaneously); similarly, different pressing forces cannot be appied to the coating heads, since the floating nature of the coating heads means that forces are balanced on opposite sides of the strip. This is unfortunate because coating speeds and pressing forces are easy to vary (i.e. in single-sided coating equipment), whereas differences of land angle and width can only be achieved by stopping the coating line and changing coating heads. Moreover, while it is easy to use coating materials of different formulation in the coating heads of two-sided coaters, the intended end use of the coated strip article may dictate the nature of the coating formulations, so there may in many cases be no freedom to choose formulations that would produce a desired thickness ratio.
The Innes patent discloses the concept of making one of the coating heads pivotable so that the angle between the land and the surface to be coated can be varied, e.g. by means of a set screw arrangement. The main problem with this is the mechanical complexity required for varying the angle without affecting the alignment of the lands. Unless the axis of rotation is coincident with the trailing edge of the land, rotation of the die to change the angle of the land will cause a component of displacement of the land in a direction parallel to the advance of the strip article. Such a displacement may cause misalignment of the coating heads and adversely affect the balance of forces. While it is conceivable to avoid such misalignment, or to provide for simultaneous rotation and translation of the land, this would be quite complex in practice.
There is therefore a need for an improved process and apparatus that will allow the coating thickness of coating films produced in two-sided direct coaters employing floating coating heads to be varied either when the apparatus is initially being set up for coating, or as the coating operation proceeds. There is a particular need for an improved process and apparatus whereby thickness ratios of coatings on opposite sides of strip articles may be varied easily and accurately during two-sided direct coating as the coating operation proceeds. Also, there is often a need to enable coating layers of the same thickness to be produced when different coating materials must be applied to opposite sides of the strip, or when coating heads of different dimensions are used on opposite sides of the strip article.
An object of the invention is to provide a method and apparatus for double-sided coating of sheet articles with a coating film whereby the coating thickness may be adjusted when required by simple and convenient means.
According to one aspect of the invention, there is provided a process of producing an elongated coated strip article having a layer of coating material on each opposite surface of the strip article, each layer having a layer thickness and the coated strip article having a ratio of said layer thicknesses at any point on the strip article, the process comprising simultaneously applying layers of solidifiable liquid coating materials on said opposite surfaces of the strip article by advancing the strip article in a direction along a path between opposed coating heads, at least one of which is a floating coating head, having material delivery slots and metering lands for delivery and metering of the liquid coating materials to said opposite surfaces to form said layers, wherein said ratio of said layer thicknesses is adjusted when required by varying said path of said strip article between said floating coating heads to cause changes in angles formed between said strip surfaces and adjacent metering lands on opposite sides of the strip article.
According to another aspect of the invention, there is provided a coating apparatus for simultaneously coating both opposed surfaces of an elongated strip article to form coating layers each having a layer thickness, the coated strip article having a ratio of said layer thicknesses at any point on the strip article, said apparatus comprising a pair of coating heads, at least one of which is a floating coating head, having delivery slots for solidifiable liquid coating material and metering lands for metering the liquid coating materials to form said layers, and a drive for advancing said elongated strip article in a direction along a path between said coating heads, said apparatus including an a deflection apparatus for varying said path of strip article advance between said floating coating heads to cause changes in angles formed between said strip article surfaces and adjacent metering lands on opposite sides of the strip article, thereby enabling adjustments of said ratio of said layer thicknesses when required.
The path of the strip article is generally the plane followed by the strip article as it advances between the coating heads. This plane can, according to the present invention, be rotated about a notional transverse axis normally, but not necessarily, located approximately between the trailing (downstream) edges of the coating heads.
The way in which the variation of the path angle of the strip article causes changes in the coating thickness ratio can be quite complex. The complexity may be best dealt with by following one of three possible approaches, as follows:
1) The path angle for providing a particular thickness ratio is determined either by computation or advance experimentation and accurately set in advance without computer control.
2) The path angle is accurately set in advance but with computer control to maintain a precise angle during production.
3) The path angle is not accurately set in advance but is entirely under computer control during production.
The invention makes it possible to adjust the ratio of coating layer thicknesses without having to replace, rotate or displace the coating heads or coating lands of the coating equipment. Mechanical and logistical complexity are therefore reduced.